One possible issue may be with vanity gTLDs like apple, ebay etc. Some expect that every Fortune 1.000.000 company will apply for its own TLD. My guess is rather the Fortune 1.000 for a start, but this does not change the nature of the issue, ie. those companies may want to use email addresses like user@tld.

The current standard is defined in RFC 2821 as such:

2.3.5 Domain

A domain (or domain name) consists of one or more dot-separated components.
[…]
The domain name, as described in this document and in [22], is the entire, fully-qualified name (often referred to as an “FQDN”). A domain name that is not in FQDN form is no more than a local alias. Local aliases MUST NOT appear in any SMTP transaction.

Hence, if either the mail client or the MTA expect to see a dot in the domain name and there is none, its behaviour may be unpredictable. The new gTLD context is addressed in the draft RFC2821bis, which states:

2.3.5. Domain Names

A domain name (or often just a “domain”) consists of one or more components, separated by dots if more than one appears.

There may be a lot of software out there that would treat user@tld as a local e-mail address (i.e. not a Fully Qualified Domain Name). It is not unusual to still find inside company data centers old internal SMTP gateways which have been quietly doing their job for a long time and were not updated for years.

Some pointed out on the IETF list and elsewhere that we have had for 10 years a ccTLD that accepts e-mail in the form of user@ai. It is one thing that the behaviour of a small ccTLD apparently generated no complaints. It is another that a large number of companies may want to force the Internet to adapt to their advertsing strategy. At this stage, we have no meaningful statistical evidence that the currently deployed software is able successfully deal with e-mail addresses that are directly under a TLD. I am not aware of any study by ICANN’s SSAC on that matter.

In any case, when ICANN will go into an agreement with the registries operating the new gTLDs, it has to be very clear that compliance with existing technical standards is a must, and not respecting them would be a breach of contract.

It would be problematic for the end users/customers/consumers if companies started advertsing e-mail addresses like support@mycompany , if the delivery of the e-mail depends on the ability of some software to be non-standards compliant.

On a related note, my colleague Franck Martin pointed out to me last Friday that browsers usually append “.com” to any domain name they consider incomplete. Again, this is going to break a lot of software that have hard-coded lists of TLDs. Similarly, there are also millions of web forms out there that check for malformed e-mail addresses based on the presence of a dot and/or hard-coded lists of TLDs.